This invention relates to halogen exchange reactions involving haloaromatic compounds and alkali metal fluorides, and more particularly to improved processes for producing polyfluorinated aromatics by catalyzed halogen exchange reactions, and to industrially important applications of such process technology.
Halogen exchange reactions for fluorinating haloaromatic compounds using alkali metal fluorides have been extensively studied heretofore. Typically they involve the reaction of a chloroaromatic compound with potassium fluoride, rubidium fluoride or cesium fluoride by heating the reactants to extremely high temperatures (above about 400xc2x0 C.) in the absence of an ancillary diluent or solvent, or by conducting the reaction at temperatures of around 200xc2x0 to 230xc2x0 C. in an aprotic solvent such as sulfolane. It has also been reported that organic fluorine compounds such as pentafluorobenzonitrile, tetrafluoro-phthalonitriles and pentafluoropyridine can be formed by reacting a corresponding chloro- or bromo-substituted compound with alkali metal halide such as potassium fluoride in benzonitrile as solvent at 190xc2x0 C. to 400xc2x0 C. in a sealed autoclave under autogenous pressure.
Use of catalysts in some exchange reactions has also been studied. Such catalysts have included quaternary ammonium salts, metal carbonyls, crown ethers and cryptates.
In most cases, the halogen exchange reaction is sluggish and tends to form product mixtures in which yields of polyfluorinated aromatics are relatively low, especially if the haloaromatic compound used is a polyhaloaromatic compound free from activating functionality such as nitro or carbonyl. For example, with hexachlorobenzene and potassium fluoride, typical product mixtures contain a mixture of co-products including hexafluorobenzene together with various chlorofluorobenzenes.
A need presently exists for a commercially feasible process whereby the halogen exchange reaction as applied to a wide variety of haloaromatic compounds may be conducted in large scale reaction equipment under relatively mild reaction conditions while providing commercially acceptable yields of the desired products. In addition, a particularly welcome contribution to the art would be the provision of a process whereby fluorinated perhaloaromatic compounds such as chloropentafluorobenzene, bromopenta-fluorobenzene, and hexafluorobenzene can be produced on a large scale in good yield under relatively mild reaction conditions, thereby making possible the more efficient, lower cost production of a variety of industrially important end products, especially polymerization catalysts, and intermediates for producing such catalysts.
This invention is deemed to fulfill these needs most expeditiously.
This invention provides a new catalytic halogen exchange reaction using an alkali metal fluoride as the fluorine source. The process enables production of a wide variety of fluorinated aromatic compounds under relatively mild reaction conditions. Moreover, the process is applicable to use as starting materials of haloaromatic compounds containing one or more halogen atoms other than fluorine, including compounds which are devoid of activating groups, as well as compounds which possess one or more activating groups in the molecule. In fact, the process is especially well adapted for polyfluorination of perhaloaromatic compounds such as hexachlorobenzene, hexabromobenzene, pentachloro-fluorobenzene, tetrachlorodifluorobenzene, trichlorotrifluorobenzene, dichlorotetrafluorobenzene, etc., which have no activating group in the molecule. In addition, the catalyzed process can be conducted with smaller excesses of the alkali metal fluoride than generally required in prior processes.
More particularly, there is provided pursuant to one of the embodiments of this invention a process which comprises (A) heating a mixture formed from ingredients comprising (i) at least one finely-divided alkali metal fluoride having an atomic number of 19 or more, (ii) at least one perhalobenzene of the formula C6FnX6-n where n is 0 to 4, and each X is, independently, a chlorine or bromine atom, and (iii) an aminophosphonium catalyst, at one or more reaction temperatures at which chloropentafluorobenzene or bromopentafluorobenzene is formed; (B) recovering chloropentafluorobenzene or bromopentafluorobenzene formed in (A); and (C) converting chloropentafluorobenzene or bromopentafluorobenzene from (B) into a pentafluorophenyl Grignard reagent or a pentafluorophenyl alkali metal compound.
In another embodiment, the above process further comprises (D) converting pentafluorophenyl Grignard reagent or pentafluorophenyl alkali metal compound from (C) into a pentafluorophenyl boron compound by reacting the pentafluorophenyl Grignard reagent or pentafluorophenyl alkali metal compound with a boron trihalide or an etherate complex thereof.
Yet another embodiment of this invention further comprises (E) converting pentafluorophenyl boron compound from (D) in a suitable solvent or diluent into a single coordination complex comprising a labile tetra(pentafluorophenyl)boron anion.
Still another embodiment of this invention further comprises (E) contacting such pentafluorophenyl boron compound from (D) with a metallocene of the formula LMX2 wherein L is a derivative of a delocalized pi-bonded group imparting a constrained geometry to the metal active site and contains up to 50 non-hydrogen atoms, M is a Group 4 metal, and each X is, independently, hydride, or a hydrocarbyl, silyl, or germyl group having up to 20 carbon, silicon, or germanium atoms under conditions to form a catalyst having a limiting charge separated structure of the formula LMX⊕ XAxe2x8ax96 wherein A is an anion formed from said pentafluorophenyl boron compound.
The substantial improvements made possible by this invention are brought about at least in part by use of an aminophosphonium catalyst in the halogen exchange reaction. As an example of such improvements, comparative studies on a 50-liter scale have shown that in reactions using hexachlorobenzene and potassium fluoride to form chloropentafluorobenzene and hexafluorobenzene, the inclusion of the aminophosphonium catalyst, tetrakis(diethylamino)phosphonium bromide, in the reaction mixture resulted in the following yield improvements:
a) Yields of desired products based on raw material inputs were increased from 12% to 25%.
b) Yields of desired products based on hexachlorobenzene input were increased from 35% to 95%.
c) Molar yields of desired products were increased from 49% to 86%.
In conducting the halogen exchange process, an agitated mixture formed from ingredients comprising (i) at least one finely-divided alkali metal fluoride, (ii) at least one haloaromatic compound having on an aromatic ring at least one halogen atom of atomic number greater than 9, such haloaromatic compound being devoid of any activating functional group on the aromatic ring to which the halogen atom of atomic number greater than 9 is bonded, and (iii) an aminophosphonium catalyst, is heated at one or more reaction temperatures at which at least one such halogen atom of the haloaromatic compound is replaced by a fluorine atom. It is particularly preferred to use as the initial haloaromatic ingredient to be subjected to the halogen exchange processing, one or more haloaromatic compounds that are not only devoid of any activating functional group on the aromatic ring to which the halogen atom of atomic number greater than 9 is bonded, but in addition have no hydrogen atom on that aromatic ring.
Another preferred embodiment includes conducting the halogen exchange reaction such that the essentially anhydrous agitated mixture when heated to one or more reaction temperatures is predominately a mixture of solids dispersed in a continuous liquid phase. Operations wherein the continuous liquid phase comprises at least one halogen-free, polar, anhydrous aprotic solvent constitute additional preferred embodiments of this invention.
Preferred catalyst ingredients for use in the various process embodiments of this invention are tetra(dihydrocarbylamino)phosphonium halides.
These and other embodiments, features and advantages of this invention will be further apparent from the ensuing description, accompanying drawing, and appended claims.